1. Field of the Invention
The present invention relates to the purification of a crude alkali carbonate containing a fluorine compound (throughout the specification, the term "alkali carbonates" will refer to alkali carbonates, bicarbonates and sesqui-carbonates). More particularly, it relates to the purification of a crude alkali carbonate which comprises separating an impurity, especially a fluorine compound, from an alkali carbonate by a novel technique.
2. Description of the Prior Art
Alkali carbonates have been used as raw material sources of alkali for various industries including the chemical industry. Accordingly, they are required to be of high purity in order to be useful for many of the various applications. They have been produced either by synthesis or by extraction from natural alkali sources. Among the alkali carbonates, sodium carbonate, often referred to as soda ash, has been used in large amounts. It has been supplied mostly by synthetic production. However, recently, more than ten percent of the world's supply of sodium carbonate has been produced from natural soda. This trend in production from the natural soda appears to be increasing.
The natural alkali sources which have been tapped most often are trona ore, containing a main component of sodium sesquicarbonate as an underground mineral (vein); alkali crust layers containing a main component of sodium sesquicarbonate and/or sodium carbonate monohydrate, which is crystallized on alkali brine lake surfaces by natural concentration; brine; and the mineral spring water containing alkali bicarbonate. These natural sodas contain undesirable impurities in addition to their main components of alkali carbonates. The natural soda produced from volcanic zones usually contains relatively large amounts of fluorine compounds, whereby it is almost impossible to use the raw natural soda for general applications except in special cases. For example, a large amount of alkali carbonates is used as a raw material for production of pharmaceuticals and foods. The contamination by impurities, especially fluorine compounds, is hazardous and toxic for the human body. Moreover, in the glass industry, by which more than half of the soda ash produced has been consumed, the presence of a fluorine compound causes various disadvantages such as the inhibition of the production of transparent glass, damage to tanks and fusing furnaces, (i.e., the corrosion of the refractory brick of the furnace) and air pollution caused by the discharge of volatile fluorine compounds which are quite hazardous and toxic. It is also preferable to use alkali carbonates free from fluorine compounds as raw materials or additives for chemicals, inorganic compounds, detergents, paper, pulp and the like. Heretofore, it has been difficult to directly separate a fluorine compound from the alkali carbonates without changing the specific characteristics of the alkali carbonates. Thus, separation of the fluorine compounds, especially the water-soluble fluorine compounds from the alkali carbonates, has not been successfully performed on an industrial basis. Consequently, such a fluorine-removal process is sorely needed.